package LeetCode;


import java.util.HashMap;
import java.util.List;

public class No141 {
    public boolean hasCycle(ListNode head) {
        if(head==null||head.next==null){
            return false;
        }

        ListNode slow = head;
        ListNode fast = head.next;
        while(slow!=fast){
            if(fast==null||fast.next==null){
                return false;
            }
            slow=slow.next;
            fast = fast.next.next;
        }
        return true;


//        HashMap<ListNode,Integer> map = new HashMap<>();
//        int n = 0;
//        while(head!=null){
//            if(map.containsKey(head)) return true;
//            map.put(head,n++);
//            head=head.next;
//        }
//        return false;
    }
}
